1. Field of the Invention
The present invention relates to a radio telecommunication apparatus used in a radio telecommunication system, such as a motor vehicle radio telephone system or a portable radio telephone system. More particularly, the invention relates to a radio telecommunication apparatus and method capable of providing information to a user on the remaining energy of a battery in the apparatus.
2. Description of the Related Art
Recently radio communication apparatus have been reduced in size and weight. For this reason, a small and light battery is required in such apparatus. Some small and light batteries are known. One of such small and light batteries is a lithium ion battery, referred to herein as a Li-ion battery. This battery has more energy density per volume and weight than a Nickel-Cadmium battery (referred to herein as a Ni-Cd battery) used more frequently in the past. In addition the Li-ion battery has another feature. FIG. 7 shows the variation of the battery voltage of Li-ion and Ni-Cd batteries in response to battery consumption time, if the consumption current flowing in the battery is constant. Referring to FIG. 7, the Ni-Cd battery voltage is substantially constant during discharge, while the Li-ion battery voltage decreases as the discharge continues. Since the Li-ion battery voltage decreases in proportion to the Li-ion battery consumption time, an apparatus with a Li-ion battery can derives a remaining consumption time by subtracting the elapsed consumption time from a total consumption time. The remaining consumption time corresponds to the remaining energy of the battery. Therefore, the apparatus may recognize the remaining energy of a Li-ion battery by detecting the Li-ion battery voltage in proportion to the Li-ion battery consumption time.
However, if this method of detecting the remaining energy of the battery is applied to a digital radio communication apparatus, the following error occurs.
A digital radio communication apparatus utilizes a time division multiple access system (called a TDMA system) as a communication system. FIG. 8 shows the basic concept of the TDMA system. Referring to FIG. 8, a base station BS sends a signal to portable radio apparatus PS1, PS2, PS3 in a service area E. The signal consists of a plurality of slots. Three slots constitute a frame. The signal is sent over a downward radio frequency channel. Each portable apparatus PS is assigned one slot of three slots. Each portable apparatus receives a downward signal during a period corresponding to the downward assigned slot. Therefore, the apparatus extracts information included only in the downward assigned slot. The portable apparatus PS sends an upward signal to the base station BS over an upward radio frequency channel. Each portable apparatus sends the upward signal during a period corresponding to an upward assigned slot. Therefore, the upward signal is intermittent. FIG. 9 shows upward and downward slots between the base station BS and the portable apparatus PS1. Referring to FIG. 9, the portable apparatus PS1 sends the intermittent upward signal to the base station BS using the upward slot #1. Further, the portable apparatus PS1 receives the downward signal during a period corresponding to downward slot #1. The upward and the downward slots #1 are referred to herein as the "transmit" slot and the "receive" slot. During a period corresponding to the downward slot #2, the apparatus PS1 sends and receives no signal. Therefore the slot corresponding to the slot #2 is referred to herein as an "idle" slot. The above three slots repeat in every frame.
FIG. 10 shows a block diagram of a conventional apparatus including the battery. Referring to FIG. 10, the battery has an internal resistance R. The battery is coupled to a battery circuit providing a predetermined voltage to each section and further coupled to an A/D convertor for converting an analog value of an output to a digital value thereof. The digitized battery voltage value V.sub.DET is provided to a control circuit. In this case, when the real voltage of the battery is defined as V.sub.Li and the current flowing through the battery as I.sub.Li, the digitized battery voltage V.sub.DET is expressed by a following expression. EQU V.sub.DET =V.sub.Li -RI.sub.Li
According to the above expression, the digitized battery voltage V.sub.DET varies in accordance with the current I.sub.Li flowing through the battery.
In the portable digital apparatus, an average current flowing through the battery during the transmit slot period is from 450 mA to 750 mA, while an average current flowing through the battery during the receive slot period and the idle slot period is 150 mA. That is, the current I.sub.Li during the period corresponding to the transmit slot is much higher than that during the periods corresponding to the receive and idle slots. Therefore, referring to the expression (1), the Li-ion battery voltage V.sub.DET during the period corresponding to the transmit slot is much less than that during the periods corresponding to the receive and idle slots. FIG. 11 shows the relationship between the digitized Li-ion battery voltage V.sub.DET and the consumption time of the battery provided in the portable digital telecommunication apparatus. Referring to FIG. 11, the digitized battery voltage V.sub.DET during the period corresponding to the transmit slot T in a frame is much less than that during the period corresponding to the receive slot R in the frame and the idle slot I in the frame. For example, assume that, the digitized battery voltage V.sub.DET is V1(v) at the time T1 and the digitized battery voltage V.sub.DET is V2(v) at the time T2 after T1. A predetermined criterion is established to estimate remaining battery energy. If the above estimation is made between V1(v) and V2(v) to determine the remaining battery energy using the criterion, the following error occurs. The remaining energy of the battery at the time T1 is estimated to be lower than the predetermined criterion, while the remaining energy of the battery at the time T2 after T1 is estimated to be higher than the criterion. That is, in spite of the actual consumption of the battery, the remaining energy of the battery is incorrectly estimated at too high a level. This kind of error occurs if the portable digital communication apparatus derives the remaining energy of the battery from the detection of the digitized battery voltage V.sub.DET at any time.
Further, the battery consumption of current during the period of time corresponding to the transmit slot varies in accordance with a position of the portable apparatus in the service area E. This is because the transmission power of the portable apparatus varies according to the distance of the apparatus from the base station BS as shown in FIG. 12. When the portable apparatus PS is near the base station BS, the transmission power of the apparatus is small. When the apparatus PS is far from the base station BS, the transmission power of the apparatus is large. The transmission power has several values. As shown in FIG. 11, if the transmission power at the time T1 is higher than the transmission power at the time T3, and the battery voltage V.sub.DET at the time T3 is V3(v), a following error occurs. In spite of the passing of time and the reduction of battery energy, the battery voltage V.sub.DET is estimated to be increased between V1 to V3. This results in an estimate that the remaining energy of the battery is higher instead of lower.
Thus, if the battery voltage is detected during a time period including the transmit slot, the above-described error occurs.